CHAPTER VIII 
LIVING PARTS OF THE STEM; WORK OF THE STEM 
91. Active Portions of the Stems of Trees and Shrubs. — 
In annual plants generally and in the very young shoots 
of shrubs and trees there are stomata or breathing pores 
which occur abundantly in the epidermis, serving for the 
admission of air and the escape of moisture, while the 
green layer of the bark answers the same purpose that is 
served by the green pulp of the leaf (Chapter XIII). 
For years, too, the spongy lenticels, which succeed the 
stomata and occur scattered over the external surface of 
the bark of trees and shrubs, serve to admit air to the 
interior of the stem. The lenticels at first appear as 
roundish spots of very small size, but as the twig or shoot 
on which they occur increases in diameter the lenticel 
becomes spread out at right angles to the length of the 
stem, so that it sometimes becomes a longer transverse slit 
or scar on the bark, as in the cherry and the birch. But 
in the trunk of a large tree no part of the bark except the 
inner layer is alive. The older portions of the bark, such 
as the highly developed cork of the cork-oak, from which 
the ordinary stoppers for bottles are made, sometimes cling 
for years after they are dead and useless except as a pro- 
tection for the parts beneath against mechanical injuries 
or against cold. But in many cases, as in the shell-bark 
hickory and the grapevine, the old bark soon falls off in 
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